This invention concerns coating systems comprising non-isocyanate and isocyanate components in organic solvents. The non-isocyanate components being an oligomer or blend of oligomers containing at least two functional groups, at least one being epoxy; optionally present is a polyester or oligo-ester or acrylic polymer having at least two hydroxyl groups.
U.S. Pat. No. 5,215,783 discloses a process for coating a substrate with a waterborne basecoat and a clearcoat containing a polymeric epoxy group.
It has now been discovered that oligomeric epoxies will react directly with isocyanates to form well-crosslinked coatings. This reaction occurs rapidly at elevated temperatures but relatively slowly at room temperature. This is in sharp contrast to polymeric acrylic epoxies which react poorly at any temperature. The room temperature reaction is enhanced significantly by the use of epoxy compounds which also include hydroxyl moieties. These epoxies can be used as diluents in traditional hydroxyl/ isocyanate coatings. This crosslinking system results in coatings with very low volatile organic content (VOC) that are durable and display good etch and mar resistance.
The invention specifically concerns a curable coating composition of a binder in organic solvent comprising
A) a non-isocyanate component wherein:
i) 5-100% by weight of the non-isocyanate component is an oligomer or blend of oligomers with a weight average molecular weight not exceeding about 3,000, a polydispersity not exceeding 1.7, containing at least two functional groups with at least one being an epoxy group, the remaining being epoxy or hydroxyl;
ii) 0-95% by weight of the non-isocyanate component of a polyester, oligo-ester or acrylic polymer each having at least two hydroxyl groups; and
B) an oligomeric isocyanate crosslinker containing at least two isocyanate groups; the equivalents of B to A being 0.5 to 3.0 of isocyanate to epoxy or epoxy plus hydroxyl.
Contemplated embodiments of the invention are those wherein component (ii) is absent and cure is accelerated by ambient moisture, and where component (ii) contains at least one hydroxyl group derived from acrylates and/or methacrylates, and at least one epoxy group derived from glycidyl methacrylate and/or glycidyl acrylate.
Also contemplated is the above composition cured at ambient conditions or baked at elevated temperatures. Such composition can include hydroxyl and/or epoxy-functional nonaqueous dispersions, and these optional crosslinkers: aldimines, ketimines, and polyaspartic esters. Catalysts such as tin and tertiary amines (alone or in combination with acetic acid) can be employed. The disclosed composition is useful in clearcoats and pigmented compositions to coat substrates, preferably vehicle bodies and vehicle body parts.
The compositions of this invention show a remarkable combination of wet-properties and film-properties. The combination of oligomeric epoxies crosslinked by oligomeric isocyanates have shown
1) the potential for extremely low volatile organic content (VOC). VOC""s below 2.0 lbs/gallon, (0.24 kg/liter) and in some cases (with only epoxy/isocyanate) approaching 1.0 (0.12 kg/liter) VOC, have been successfully sprayed with excellent appearance and cure;
2) the etch resistance of these coatings is superior to standard hydroxyl/isocyanate systems of similar film Tg (glass transition temperature). This results in coatings with a superior etch/mar balance which is critical to today""s finishes;
3) the fracture properties of these systems, as measured by single indentor testing, is superior to standard hydroxyl/isocyanate systems; and
4) excellent durability exceeding 7000 hours of acccelerated QUV exposure (using an FS-40 bulb) has been seen with these coatings.
These epoxy or epoxy/hydroxyl-functional oligomers can be used to improve the spray solids or film properties of standard polymeric isocyanate crosslinked systems.
Binder Components
Representative binder components of these systems include epoxy functional oligomers, epoxy/hydroxyl-functional oligomers and isocyanate functional oligomers. Other functional oligomers and polymers can also be included in the formulations of this invention.
Component A(i)
The oligomeric component contains at least two functional groups and should have a molecular weight of less than about 3000. Typical epoxy components containing a hydroxy functionality or (OH) group include, among others, sorbitol polyglycidyl ether, mannitol polyglycidyl ether, pentaerythritol polyglycidyl ether, glycerol polyglycidyl ether, low molecular weight epoxy resins such as epoxy resins of epichlorohydrin and bisphenol-A, and polyglycidyl ethers of isocyanurates, for example, xe2x80x9cDenecolxe2x80x9d EX301 from Nagase and DCE-358(copyright) sorbitol polyglycidyl ether from Dixie Chemical. These types of oligomers are preferred for ambient cure, but are also useful for baked systems.
Epoxy components which typically do not contain significant hydroxy functionality include, among others, di- and polyglycidyl esters of polycarboxylic acids, and di- and polyglycidyl esters of acids, such as Araldite CY-184(copyright) from Ciba-Geigy, or XU-71950 from Dow Chemical are preferred since they form high quality finishes. Cycloaliphatic epoxies can also be used, such as ERL-4221 from Union Carbide. These oligomers are primarily used in baked systems, but can be used at low levels in ambient cured systems.
Component B
The composition also contains an organic isocyanate crosslinking agent in the amount of 0.5 to 3.0 equivalents of isocyanate per equivalent of epoxy or epoxy/hydroxyl. Optimum film properties are achieved when one epoxy group reacts with two isocyanate groups. However, it was determined that a broad latitude in stoichiometry of isocyanate to epoxy can sometimes be useful depending on final wet- and dry-coating properties desired. Any of the conventional aromatic, aliphatic, or cycloaliphatic isocyanates; trifunctional isocyanates and isocyanate functional adducts of a polyol and a diisocyanate can be used. Typically useful diisocyanates are 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, toluene diisocyanate, bis-cyclohexyl diisocyanate, tetramethylene xylene diisocyanate, ethyl ethylene diisocyanate, 2,3-dimethyl ethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-phenylene diisocyanate, 1,5-napthalene diisocyanate, bis-(4-isocyanatocyclohexyl)-methane, 4,4xe2x80x2-diisocyanatodiphenyl ether and the like.
Typical trifunctional isocyanates that can be used are triphenylmethane triisocyanate, 1,3,5-benzene triisocyanate, 2,4,6-toluene triisocyanate and the like, Trimers of diisocyanates also can be used such as the trimer of hexamethylene diisocyanate which is sold under the tradename xe2x80x9cDesmodurxe2x80x9d(copyright) N-3390 and the trimer of isophorone diisocyanate. Trifunctional adducts of triols and diisocyanates can be used.
Optional Ingredients
The present coating composition can further comprise a functional amount of catalyst, generally about 0.1 to 5 weight percent, based on the weight of solids in the formulation. A wide variety of catalysts can be used, such as dibutyl tin dilaurate or tertiary amines such as triethylenediamine. These catalysts can be used alone or in conjunction with carboxylic acids such as acetic acid. It is preferred that a catalyst be employed.
The coating compositions of the present invention are formulated into high solids coating systems dissolved in at least one solvent. The solvent is usually organic. Preferred solvents include aromatic hydrocarbons such as petroleum naphtha or xylenes; ketones such as methyl amyl ketone, methyl isobutyl ketone, methyl ethyl ketone or acetone; esters such as butyl acetate or hexyl acetate; and glycol ether esters such as propylene glycol monomethyl ether acetate. It is preferred to employ solvent.
The coating compositions of the present invention can also contain up to 40% of total binder of a dispersed acrylic component which is a polymer particle dispersed in an organic media, which particle is stabilized by what is known as steric stabilization. Hereafter, the dispersed phase or particle, sheathed by a steric barrier, will be referred to as the xe2x80x9cmacromolecular polymerxe2x80x9d or xe2x80x9ccorexe2x80x9d. The stabilizer forming the steric barrier, attached to this core, will be referred to as the xe2x80x9cmacromonomer chainsxe2x80x9d or xe2x80x9carmsxe2x80x9d.
The dispersed polymer contains about 10 to 90%, preferably 50 to 80%, by weight, based on the weight of the dispersed polymer, of a high molecular weight core having a weight average molecular weight of about 50,000 to 500,000. The preferred average particle size is 0.1 to 0.5 microns. The arms, attached to the core, make up about 10 to 90%, preferably 10 to 59%, by weight of the dispersed polymer, and have a weight average molecular weight of about 1,000 to 30,000, preferably 1,000 to 10,000.
The macromolecular core of the dispersed polymer is comprised of polymerized acrylic monomer(s) optionally copolymerized with ethylenically unsaturated monomer(s). Suitable monomers include styrene, alkyl acrylate or methacrylate, ethylenically unsaturated monocarboxylic acid, and/or silane containing monomers. Such monomers as methyl methacrylate contribute to a high Tg (glass transition temperature) dispersed polymer, whereas such xe2x80x9csofteningxe2x80x9d monomers as butyl acrylate or 2-ethylhexylacrylate contribute to a low Tg dispersed polymer. Other optional monomers are hydroxyalkyl acrylates or methacrylates or acrylonitrile. Optionally, the macromolecular core can be crosslinked through the use of diacrylates or dimethacrylates such as allyl methacrylate or post reaction of hydroxyl moieties with polyfunctional isocyanates.
The macromonomer arms attached to the core can contain polymerized monomers of alkyl methacrylate, alkyl acrylate, each having 1 to 12 carbon atoms in the alkyl group, as well as glycidyl acrylate or glycidyl methacrylate or ethylenically unsaturated monocarboxylic acid for anchoring and/or crosslinking. Typically useful hydroxy-containing monomers are hydroxy alkyl acrylates or methacrylates as described above.
Additional crosslinkers can be included in this formula such as aldemine, including the reaction product of isobutyraldehyde with diamines such as isophorone diamine and the like; ketimines such as the reaction product of methyl isobutyl ketone with diamines such as isophorone diamine; and polyaspartic esters.
The coating compositions of the present invention can also contain conventional additives such as pigments, stabilizers, ultraviolet light stabilizers, antioxidants, rheology control agents, flow agents, toughening agents and fillers. Such additional additives will, of course, depend on the intended use of the coating composition. Fillers, pigments, and other additives that would adversely effect the clarity of the cured coating will not be included if the composition is intended as a clear coating.
Component A(ii):
The coating compositions of the present invention can also an acrylic polymer of weight average molecular weight greater than 3,000, or a conventional polyester such as SCD(copyright)-1040 from Etna Product Inc. for improved properties and appearance, sag resistance, flow and leveling and such. The acrylic polymer can be composed of typical monomers such as acrylates, methacrylates, styrene and the like and functional monomers such as hydroxy ethyl acrylate, glycidyl methacrylate, or the like.
Representative hydroxyl-functional oligomers that can be employed as component A(ii) include the reaction product of multifunctional alcohols such as pentaerythritol, hexanediol, trimethylol propane, and the like, with cyclic monomeric anhydrides such as hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, and the like, said reaction product further extended by reaction with monofunctional epoxies such as butylene oxide, propylene oxide, and the like to form hydroxyl oligomers.
Non-alicyclic oligomers (linear or aromatic) can include succinic anhydride- or phthalic anhydride-derived moieites such as described above. Caprolactone oligomers which can be made by reacting caprolactone with a cycloaliphatic, aliphatic or aromatic polyol can also be used. Particulary useful caprolactone oligomers are described in columns 4 to 5 of U.S. Pat. No. 5,286,782.
Preferred oligomers A(ii) have weight average molecular weights not exceeding about 3,000 with a polydispersity not exceeding about 1.7; more referred oligomers have molecular weights not exceeding about 2,500 and polydispersity not exceeding about 1.4; most preferred oligomers have molecular weights not exceeding about 2,200, and polydisperity not exceeding about 1.25.
The coating compositions are typically applied to a substrate by conventional techniques such as spraying, electrostatic spraying, roller coating, dipping or brushing. The present formulations are particularly useful as a clear coating for outdoor articles, such as automobile and other vehicle body parts. The substrate is generally prepared with a primer and/or a color coat or other surface preparation prior to coating with the present compositions.
After application to a substrate, the present compositions can be cured by heating to a temperature of about 120xc2x0 to 150xc2x0 C. for a period of about 15 to 90 minutes or with the proper formulation can be cured at ambient conditions (about 60xc2x0 to 110xc2x0 F., depending on the geographical location, usually 65xc2x0 to 90xc2x0 F.).
The performance characteristics of the final cured coating composition are excellent, providing a combination of excellent gloss and durability to abrasion, sunlight and acidic rain. At the same time, the compositions provide low volatile organic content and ease of handling. The ability to apply the present compositions by spraying techniques with the unusually low VOC content is surprising.